Designs of threaded orthopedic fasteners have historically struggled with a transition from unthreaded to threaded portions. The manufacturing process of cutting a thread on a shank typically leaves an abrupt discontinuity between threaded and unthreaded portions, where the thread root diameter is less than the unthreaded shank diameter. Problems occur when such orthopedic fasteners are secured into bone. When repetitive loads are exerted on the secured fastener (such as are likely to be experienced in exemplary orthopedic procedures such as bone distraction or fracture repair), such fasteners become inherently susceptible to acute and/or fatigue failure at the abrupt threaded/unthreaded transition, since repetitive loads become more focused at the threaded/unthreaded discontinuity when the fastener is held rigidly.
This tendency towards failure is especially pronounced when such orthopedic fasteners are secured into cortical bone. Cortical bone, typically found on the outside of a bone, is harder than cancellous bone, typically found on the inside of a bone. This means that threads on orthopedic fasteners can typically be fastened tighter into cortical bone than cancellous bone. This in turn means that when the fastener is secured into cortical bone, it can be held more rigidly than when secured into cancellous bone, causing repetitive loads on the fastener to become yet more focused at the threaded/unthreaded discontinuity.
The orthopedic fasteners of the present invention are designed and claimed for use in cortical bone. That is not to say that they may not have serviceable applications in certain types of cancellous bone. However, the focus is on cortical bone.
The references in the previous paragraphs to cortical and cancellous bone deserve further discussion. Orthopedic fasteners are typically designed to fasten either cortical bone or cancellous bone. As noted above, cortical bone, typically found on the outside of a bone, is tougher and harder than cancellous bone, which is typically found on the inside of a bone. Cancellous bone has soft and malleable characteristics, making it useful in, for example, bone grafts. In comparison, cortical bone is considerably harder. When over-stressed, cortical bone will typically crack (at the higher stresses), whereas cancellous bone will typically deform and “pack” (at the lower stresses). As a result, these and other differences in the material properties of cortical and cancellous bone compel different thread designs for orthopedic fasteners. For, example, the thread profile provided on some types of cancellous bone fasteners is a thinner and sharper thread, designed to hold an unimpaired thickness of cancellous bone between the threads, thereby reinforcing the security of the fastener as placed in the cancellous bone. In this way, cancellous bone fasteners of this design will secure into the cancellous bone without “packing” or otherwise deforming the malleable cancellous bone between threads. In contrast, thread profiles provided on other types of cancellous bone fasteners are configured to do the opposite. Such designs are configured to intentionally “pack” the surrounding cancellous bone in order to afford better friction grip on the threads.
Published U.S. Patent Application No. 2003/0158555 (Sanders et al.) discloses conventional orthopedic fastener for use in either cortical or cancellous bone, depending on the type of accompanying threads selected, such as has been described in the previous few paragraphs. More specifically, FIG. 1a of Sanders et al. discloses an abrupt discontinuity between unthreaded and threaded portions. As noted above, the abrupt discontinuity is likely to make the fastener increasingly susceptible to repetitive load failure at the discontinuity once the fastener is secured into bone.
Some prior art designs have attempted to overcome the problems with abrupt discontinuities by providing threads whose root diameter is the same as the shank diameter. While serviceable, such designs are not preferable because of the more complicated fastener manufacturing required to provide threads whose root diameter is the same as the shank diameter upon which they are deployed. Some designs of this type are also disadvantageous in that the bending resistance of the shank is diminished by wholesale removal of metal down to a thread root diameter.
Other prior art designs have attempted to overcome the problems with abrupt discontinuities by providing transitional thread roots between unthreaded to threaded portions to smooth out the transition. Typically, such prior art thread root transitions are conical in shape. For example, U.S. Pat. No. 5,242,447 (Borzone) discloses, in FIG. 2, a threaded pin with a tapered thread root over the entire length of the threaded portion. Such an arrangement is considered disadvantageous, in that a specimen of cortical bone receiving the pin must inevitably also receive at least a portion of tapered thread root transition. In such an arrangement, the tapered thread root transition, especially near the unthreaded portion of the pin, is likely to cause unwanted radial stress on the zone of cortical bone immediately surrounding the point of entry of the pin, resulting in almost inevitable damage to the bone during insertion of the pin. Such unwanted radial stress is also likely to lead to cracking of the bone over time, especially in the presence of operational repetitive loading of the pin. Typically this repetitive-load cracking condition begins from radial stress micro-cracks caused by insertion of the pin. The micro-cracks enlarge and degenerate into substantial cracks as repetitive operational loads are placed upon the pin.
U.S. Pat. No. 6,949,100 (Venturini) discloses, in FIGS. 1 and 2, a bone screw with a tapered thread root over the entire length of the threaded portion. Such an arrangement is disadvantageous for the same reasons as discussed immediately above with respect to U.S. Pat. No. 5,242,447 (Borzone). Indeed, one disclosed embodiment in Venturini teaches a conical taper of approximately 2 mm in reduced thread root diameter over the length of the taper. Those of ordinary skill in this art will understand the radial stress exerted on cortical bone by a thread root diameter change of 2 mm to be inoperable on cortical bone, almost inevitably causing cracking of the bone at the point of entry while attempting to secure the pin. Such a thread root diameter change, coupled with the thread shape and profile disclosed by Venturini, are clearly understood by those of ordinary skill in this art to be indicative of a bone screw better suited for fixation in cancellous bone, not cortical bone.
U.S. Patent Application No. 2007/0053765 (Warnick et al.) discloses numerous styles of threads for bone screws, at least one of which (for example, in FIG. 3) provides a tapered thread root transition zone. However, those of ordinary skill in the art will understand that the tapered thread root transition zone disclosed by Warnick et al. is intended to be inserted at least partially into the bone, noting that removal of the tapered root transition from inside the bone is likely to cause re-tightening issues. Additionally, as noted above with respect to Borzone and Venturini, the considerable radial stress exerted on cortical bone at the point of entry of a tapered root transition zone into the bone renders such tapered thread root transitions highly disadvantageous when located inside cortical bone.
U.S. Pat. No. 7,198,488 (Lang et al.) discloses a dental implant suitable for receiving crowns and other false teeth, at least one embodiment of which provides a tapered thread root transition zone. However, those of ordinary skill in this art will understand that the tapered root transition zone disclosed by Lang et al. is intended to be inserted entirely into the bone.
U.S. Pat. No. 5,665,087 (Huebner) discloses, in FIG. 1, a bone screw with a tapered thread root. However, those of ordinary skill in this art will understand that the tapered thread root disclosed by Huebner is intended to be inserted entirely into the bone. Moreover, it will be seen from the disclosure of Huebner that the bone screw is designed for use in cancellous bone.
Therefore, there exists a need for an improved threaded orthopedic fastener whose discontinuity between threaded and unthreaded portions has increased fatigue life performance (i.e., is resistant to repetitive loads) when secured into cortical bone. Other pitfalls encountered by the prior art also should be avoided, such as deploying a tapered thread root transition inside the cortical bone.